Cesium contamination from Fukushima disaster found penetrating deeper into ground

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Radioactive materials from the Fukushima No. 1 power plant were found 5 cm beneath the ground three months after the crisis started and now are believed to have penetrated between 5 and 25 cm deeper, according to the Japan Times report on researchers with the Japan Atomic Energy Agency.

The hazardous materials likely seeped into the soil with rainwater, researchers with the Japan Atomic Energy Agency said Wednesday. “Further delay in decontamination work will allow the radioactive materials to sink into the ground deeper, and it will impose more burdens on those involved in the decontamination,” said Haruo Sato of the agency’s Horonobe Underground Research Center in Hokkaido.

The site farthest from the plant to have high levels of contamination, at 1.48 million becquerels of cesium per square meter, was the town of Namie, in Fukushima, located 32.5 km from the power plant, research by the Science Ministry and other sem-igovernmental bodies revealed.

Soon after the disaster, scientists at Tokyo University’s Department of Agriculture and Life Sciences began to analyze soil samples from the area to determine how radiation was spreading.   Prof Tomoko Nakanishi presented their initial findings in the Japanese journal Radioisotopes in August 2011. She says the radioactive cesium does not move very far once it bonds with the soil particles.

“The rainfall was 198mm during the first three months and the cesium… moved 21.6mm. Then in the next three months the cesium moved only 5.6mm – despite the rainfall being three times higher. This shows with time, the cesium was more firmly adhered to the soil.”

In an April 2011 interview with the New York Times, Didier Champion, director of the environmental and response division of the French Institute for Radiological Protection and Nuclear Safety said regarding cesium, decontamination “has to be done very quickly,” adding, “Cesium tends to fix to materials and into soil.” Lee Chin-shan, a professor at the School of Public Health in Taipei, also notes that cesium-137 tends to accumulate underground.

Two major downpours washed significant amounts of radiation into the soil — on March 15-16 in Fukushima and on March 21-23 in Tochigi, Ibaraki, Saitama and Chiba prefectures.    Researchers in Tokyo detected a spike in radiation levels on March 15.   Radioactive barium, cesium, iodine and tellurium were detected March 16 in a radiation plume released by damaged nuclear reactors at Fukushima. A partly dispersed cloud passed by the Tokyo area the same day, Austria’s Meteorological and Geophysics Center reported.

Highly contaminated leaf vegetables such as spinach were found soon after the nuclear crisis began, in Fukushima Prefecture and other parts of the country.  Spinach grown in Ibaraki Prefecture, for example, was found to contain 1,931 becquerels of radioactive cesium per kilogram last March 20 — almost four times higher than the government’s maximum limit.

Then as rain drizzled down on the evening of March 21, radioactive material again fell on the city. The problem in Tokyo is that the greater part of the area is so developed, radioactive material was much more likely to have fallen on concrete, and for the following week, radioactivity in the air and water dropped rapidly.

Radioactive particles were carried aloft in the jet stream across the Pacific and arrived in the United States by March 17th, and completed their first pass around the world by March 24th.

While the cesium forms a tight bond with clay particles in the soil surface, wherever cesium landed on hard surfaces – such as concrete or asphalt – it presented a different problem.  The cesium finds its way into tiny pores on the surface, and the only way to remove it effectively is to grind off the surface, a difficult procedure which can create contaminated dust.

Decontamination workers have been quoted in reports admitting they weren’t sure how to measure the uneven ground or what to do with the snow on top of it adding, “We often encounter situations that are not in the manual and wonder if we are doing the right thing,”

Lawrence Boing, manager of special projects in the nuclear engineering division at Argonne National Laboratory in Illinois, also says the decontamination project must be a top priority. “Sooner is always better when you have something that can be driven down into soils,” he said.

A study by the Norwegian Institute for Air Research (NILU) found that around 16,700 PBq of xenon-133 (250% of the amount released at Chernobyl) were emitted by the Fukushima power plant between March12th and 19th. This constitutes the largest release of radioactive Xenon in history. Additionally, the NILU study found that 35.8 PBq of caesium-137 (42% of the amount released at Chernobyl) were emitted by the Fukushima power plant between March 12th and 19th.

Decontamination efforts have become a contentious topic in both Fukushima and other highly contaminated areas outside the prefecture. Fukushima Gov. Yuhei Sato has said he will strive to rebuild the disaster-stricken prefecture swiftly to help local residents return home, nurture new industries to revitalize the badly-damaged local economy, and promote renewable energy to replace nuclear power generation.

Areas outside Fukushima also will be targeted, such as Miyagi, Saitama and Chiba prefectures.   Kawauchi, Fukushima, Date and Minamisoma have also just launched limited decontamination efforts.  At the current time there are no plans to decontaminate Tokyo


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